The present invention relates to a device for effectively preventing squeak in a disc brake apparatus during the braking operation.
It has been known that in a disc brake apparatus whose disc rotor rotates together with an axle and is pinched by friction pads operated by a hydraulic actuator or the like to brake a vehicle, a very-unpleasant high-frequency noise referred to as a squeak is generated due to the hardness of the disc rotor, and the relationship between the properties of the rotor and the pads and so forth when the rotor is pinched by the pads during the braking operation. It has been confirmed that if the friction pads are comprised of an organic material whose main constituent is asbestos, the direction of the vibration of the disc rotor and that of the processing of the wave of the vibration are coincident with each other as well as the flexural or torsional vibration of a plate, as shown in FIG. 8 , namely, the main component of of the vibration of the disc rotor is transverse vibration. FIG. 11 shows the characteristic of the transverse vibration of the disc rotor and indicates the points of resonance with regard to the transverse wave in the rotor. FIG. 12 shows the frequency and magnitude of the squeaks in the disc brake apparatus during the actual braking operation in the case that the friction pads are comprised of the organic material. It is apparent from the examination of FIGS. 11 and 12 that some of the squeaks correspond to the points of fourth-order, fifth-order and seventh-order resonance in the transverse vibration. To conventionally prevent such squeaks, an annular metal member is fitted on the peripheral surface of a disc rotor to shift the points of resonance as disclosed in the U.S. Pat. No. 3,286,788 and the GB patent No. 934,096, or a plurality of holes or grooves are provided in braking surfaces of a disc rotor to shift the points of resonance as shown in the Japanese Utility Model Application (OPI) No. 108,880/79 (the term "OPI" as used herein means an "unexamined published application").
However, since friction pads have been recently comprised of a semi-metallic or a non-asbestos material, squeaks in a disc brake apparatus cannot be prevented enough by the conventional transverse-vibration-suppressing countermeasures mentioned above. FIG. 10 shows the frequency and magnitude of the squeaks in the disc brake with the friction pads comprised of a semi-metallic or non-asbestos material. It has turned out that the relationship between the frequency and magnitude of the squeaks is very approximate to that (shown in FIG. 9) between the frequency and magnitude of the longitudinal vibration of the disc rotor of the disc brake apparatus, in which the rotor performs the vibration of expansion and contraction in the direction of the thickness thereof and the wave of the vibration proceeds in the circumferential direction of the rotor perpendicularly to the direction of the thickness thereof. It is understood through the comparison of FIGS. 9 and 10 that there are points of resonance at 8.4 kHz (second order), 12.6 kHz (fourth order) squeaks and the longitudinal vibration. As a result, it is concluded that the squeaks of the disc brake with the friction pads comprised of a semi-metallic or non-asbestos material are based on the longitudinal vibration of the disc rotor.